In certain types of signaling and communication systems, such as in vital railroad continuous code cab signaling apparatus, it is mandatory that each subsystem, portion, component, and element must operate in a fail-safe manner in order to ensure the integrity of the overall system. The coded continuous cab signal system may be divided into two basic components or subsystems, namely, the wayside portion and the car-carried portion.
In practice, the wayside portion includes a coded transmitter which couples coded alternating current signals into the running rails. The coded alternating current signals are picked up by inductive receiver coils mounted forward of the front wheels of the engine or locomotive. The picked-up signals, which are made of the carrier frequency and a selected one of a number of code rates, after being amplified and demodulated, are applied to a code following relay termed a master relay. This relay, in turn, drives the code filters, which are selectively responsive to the particular code rate. These filters drive decoding relays to provide a given cab signal indication to the trainman or engineer in the cab of the locomotive. The "master" relay is a fail-safe bipolar magnetic-stick relay designed to be responsive to low-voltage code rates produced by the amplifier section of the carcarried receiver. While the electro-magnetic type of "master" relay has operated satisfactorily and admirably in previous cab signal equipment, it is relatively bulky, heavy and expensive, and therefore it would be advantageous and desirable to replace this electromagnetic relay with a smaller, lighter and inexpensive device or element, preferably a fail-safe solid-state circuit. There are certain features which are accomplished by the electro-magnetic master relay which must also be performed by an acceptable electronic relay. Further, an acceptable electronic polarized relay circuit not only must possess the vitality of an electro-magnetic master relay but also must provide a sufficient amount of electrical power to drive the code filters of the decoding unit. In addition, the relay circuit must also safely measure the amplitude of picked-up signals for producing an output when and only when the level of the picked-up signals exceeds a predetermined level in order to prevent signals in the adjacent track rails from falsely activating a cab signal indication. Additionally, an electronic master relay must produce an output having an even duty cycle even though the recovered modulation varies in amplitude or has sloped sides.